JP2000031573A - Optical amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily adjust the gain and thereby prevent change of dependence on wavelength of the gain due to the change of gain by isolating and combining the feedback beam and signal beam of the wavelength band in the course of the rare earth doped optical fiber and then attenuating the isolated light beam with an optical attenuator. SOLUTION: One rare earth doped optical fiber is divided into a couple of rare earth doped optical fibers 1a, 1b which are excited respectively by a pumping light sources 2a, 2b. The amplified signal beam and oscillated beam are in different wavelengths in the band and the signal beam and oscillated beam are isolated to pass different optical paths using optical branching units 9a, 9b between the two amplifying stages. An optical attenuator 8 and a variable optical attenuator 10 are provided respectively in the optical path of oscillated beam and signal beam. In the range where the internal oscillation is maintained, reflection coefficient of optical fiber grating filters 5a, 5b and oscillated beam fix the gain of amplifying medium with amount of attenuation of optical attenutator 8. Meanwhile, the signal beam can adjust the gain without changing depending on wavelength by changing attenuation amount of the variable optical attenuator 10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an optical amplifier.

[0002]

2. Description of the Related Art An optical amplifier using a rare earth-doped optical fiber has excellent characteristics such as high gain, high output, and low noise, so that the performance of an optical communication system has been greatly improved. The principle of the optical amplifier is that stimulated emission caused by the inversion of the energy level of rare earth ions when excitation light having a wavelength corresponding to the excitation level of rare earth ions added to the rare earth doped optical fiber is incident on the rare earth doped optical fiber. This amplifies the signal light.

In particular, an optical fiber amplifier (hereinafter abbreviated as “EDFA”) doped with erbium (Er) has an amplification wavelength band of the lowest loss wavelength band (1.55 μm) of a silica-based optical fiber.
Band), high efficiency, and high gain and low noise amplification characteristics can be easily obtained.

When a wavelength division multiplexing transmission system is constituted by using an optical fiber amplifier such as an EDFA, a wavelength multiplexed optical signal can be amplified at a time. Can be constructed.

However, in such a wavelength division multiplexing transmission system, when the number of multiplexed channels dynamically fluctuates, the gain per wave also fluctuates.

In order to avoid such gain fluctuation,
A method has been proposed in which the signal light intensity input to the optical fiber amplifier and the amplified output signal light intensity are monitored, and the pump light intensity is controlled so that the ratio between them is constant.

However, in this method, in addition to the complicated structure, it is difficult to stabilize the gain with high accuracy over a wide input range, and there is a problem in the transient response characteristic to the time variation of the input signal light power. There is.

As a method of solving such a problem, a method of stabilizing a gain by causing oscillation in an amplifier has been proposed.

FIG. 4 is a block diagram showing a conventional example of an optical amplifier.

As in the ordinary optical fiber amplifier, its basic parts are a rare-earth-doped optical fiber 1, a pump light source 2 for inputting pump light into the rare-earth-doped optical fiber 1, and an optical multiplexer for multiplexing signal light and pump light. A demultiplexer 3 and optical isolators 4a, 4a connected to the input and output sides of the optical amplifier to prevent reflected return light.
b. Optical fiber grating filters 5a and 5b are connected to both ends of the rare earth-doped optical fiber in order to cause oscillation in the optical amplifier.

The optical fiber grating filter 5a,
Reference numeral 5b shows a diffraction grating formed in the optical fiber by irradiating a high-output short-wavelength laser beam to the optical fiber containing germanium at a high concentration by a holographic method. By using such a filter, it is possible to reflect only an optical signal having a desired wavelength and a desired bandwidth by changing the period of the diffraction grating, the number of gratings, and the like. There is an advantage that a loss due to connection or the like is small as compared with the case where it is configured.

In the conventional optical amplifier shown in FIG. 4, oscillation occurs at the wavelength selected by the optical fiber grating filters 5a and 5b, and the gain of the amplifier is fixed. The magnitude of the gain is determined so that the gain of the feedback loop composed of the amplifying unit and the reflecting unit is “1”, and thus is determined by the reflectance of each of the optical fiber grating filters 5a and 5b.

According to such a method using internal oscillation, a constant gain (signal light output Po) can always be maintained irrespective of a change in the signal light input Pi. Transient response characteristics are also stable. Moreover, complicated electrical control is not required.

FIG. 6 is a diagram showing the difference in the characteristic of the dependence of the gain on the input signal light power depending on the presence or absence of optical feedback. In the figure, the horizontal axis indicates the input signal light power, and the vertical axis indicates the gain.

As shown in FIG. 1, when there is no optical feedback control (oscillation), the gain greatly fluctuates depending on the input signal light power. However, when there is optical feedback control, the gain is stabilized at a constant value. The input signal power range is extended. In addition, if there is no optical feedback control, the wavelength dependence of the gain changes according to the input signal light power.However, by performing control using a feedback loop, the wavelength dependence of the gain is limited in the range where the gain is stabilized. The nature is also stabilized.

FIG. 5 is a block diagram showing another conventional example of an optical amplifier.

This optical amplifier employs two optical circulators 6a and 6b having forward characteristics from terminal A to terminal B and from terminal B to terminal C.

An amplifying section comprising an excitation light source 2, a rare earth-doped optical fiber 1 for amplifying light by the excitation light from the excitation light source 2, and an optical multiplexer / demultiplexer 3 for multiplexing the signal light and the excitation light. The terminal B of the optical circulator 6a is connected to the input side,
The terminal B of the optical circulator 6b is connected to the output side of the amplifier. An optical attenuator 8 and an optical bandpass filter 7 are connected in series between a terminal C of the optical circulator 6a and a terminal A of the optical circulator 6b.

In such an optical amplifier, the signal light input Pi reaches the terminal B from the terminal A of the optical circulator 6a, and is multiplexed with the pumping light of the pumping light source 2 by the optical multiplexer / demultiplexer 3, and the rare-earth-doped optical fiber 1 After being amplified, it is output from the terminal C via the terminal B of the optical circulator 6b.

On the other hand, a terminal C of the optical circulator 6a,
Since the connection with the terminal A of the optical circulator 6b is connected via the optical bandpass filter 7 and the optical attenuator 8,
The signal light input Pi reaches the terminal C from the terminal B of the optical circulator 6a, and the optical attenuator 8 and the optical bandpass filter 7
, A feedback loop is formed from the terminal A of the optical circulator 6b to the terminal B, and returns to the terminal B of the optical circulator 6a via the amplifying unit.

Therefore, oscillation occurs at the wavelength selected by the optical band-pass filter 7, and the gain of the amplifying section composed of the rare-earth-doped optical fiber 1 and the pumping light source 2 becomes constant. The fixed gain can be adjusted by the optical attenuator 8.

Since such a feedback loop is in the opposite direction to the signal light path, the internal oscillation light is not output outside the amplifier. Therefore, any wavelength including the signal light wavelength band can be selected as the oscillation wavelength of the internal oscillation light.

[0023]

When an optical amplifier is used in an optical communication system, it is necessary to adjust a gain according to a loss in a transmission system. However, in the conventional optical amplifier shown in FIG. 4, the gain is determined by the reflectance of the fiber grating filter and is difficult to adjust. Further, even if the reflectivity can be changed by some method or the loss of the return optical path can be easily changed by another optical feedback configuration, the wavelength dependence of the gain changes with the change of the set gain. The problem remains.

On the other hand, it is possible to provide an optical attenuator at the input or output of the optical amplifier and adjust the attenuation factor to change the total gain. In this case, however, excessive loss causes deterioration in noise characteristics and output. This is undesirable because it causes a reduction in optical power.

The conventional optical amplifier shown in FIG. 5 has the same problem.

An object of the present invention is to solve the above-mentioned problems and to provide an optical amplifier which can easily adjust the gain and does not cause a change in the wavelength dependence of the gain with the change of the gain.

[0027]

To achieve the above object, an optical amplifier according to the present invention comprises a pump light source, a rare earth-doped optical fiber for amplifying light by pump light from the pump light source, and a part within an amplification band. And feedback means for selectively returning light in the wavelength band to the rare-earth-doped optical fiber, and separating and combining the return light and signal light in the wavelength band in the middle of the rare-earth-doped optical fiber. And an optical attenuator for attenuating the separated signal light.

In addition to the above configuration, the feedback means of the optical amplifier of the present invention is preferably an optical fiber grating filter disposed at both ends of the rare-earth-doped optical fiber and reflecting light in a wavelength band into the rare-earth-doped optical fiber.

In addition to the above configuration, the feedback means of the optical amplifier according to the present invention includes an output-side optical coupler disposed at one end of the rare-earth-doped optical fiber for extracting amplified light, and a feedback means disposed at the other end of the rare-earth-doped optical fiber. And a band-pass filter provided between the two optical couplers and transmitting light in a wavelength band between the two optical couplers.

In addition to the above configuration, the feedback means of the optical amplifier according to the present invention includes an output-side optical circulator disposed at one end of the rare-earth-doped optical fiber for extracting amplified light, and a feedback means disposed at the other end of the rare-earth-doped optical fiber. And a band-pass filter provided between the two optical circulators for transmitting the light in the wavelength band.

In addition to the above configuration, the separating / combining means for separating / combining the return light and the signal light in the wavelength band in the middle of the rare-earth-doped optical fiber of the optical amplifier of the present invention includes an input device disposed at one end of the rare-earth-doped optical fiber. Side optical circulator and an output-side optical circulator arranged at the other end of the rare-earth-doped optical fiber, and the return light and signal light in the wavelength band propagate through the rare-earth-doped optical fiber in opposite directions via both optical circulators. You may make it.

In addition to the above configuration, the separating / combining means for separating / combining the feedback light and the signal light in the wavelength band on the way of the rare-earth-doped optical fiber of the optical amplifier according to the present invention provides the light according to the wavelengths of the feedback light and the signal light. May be an optical multiplexer / demultiplexer that separates / combines.

In addition to the above configuration, the optical amplifier of the present invention has control means for detecting a part of the amplified output signal light and controlling the attenuation of the attenuator so that the detected power becomes constant. Is also good.

In addition to the above configuration, the optical amplifier of the present invention detects the optical power of a specific wavelength component in the amplified output signal light, and controls the amount of attenuation of the optical attenuator so that the detected power becomes constant. May be provided.

According to the present invention, the signal light and the oscillating light are separated in the middle of the rare-earth-doped optical fiber, and only the signal light is selectively attenuated. The gain can be adjusted without changing the characteristics.

[0036]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1 is a block diagram showing an embodiment of the optical amplifier of the present invention.

The basic structure and operation are the same as those of the conventional example shown in FIG.

The optical amplifier shown in FIG. 1 divides one rare-earth-doped optical fiber into two, a rare-earth-doped optical fiber 1a and a rare-earth-doped optical fiber 1b.
Excited by excitation light from a and 2b. The signal light to be amplified and the oscillation light are in different wavelength bands within the amplification band,
Between the two amplification stages, the signal light and the oscillating light are separated using optical multiplexers / demultiplexers 9a and 9b so as to pass through different optical paths. An optical attenuator 8 and a variable optical attenuator 10 are provided on the optical paths of the oscillation light and the signal light, respectively. The variable optical attenuator 10 provided on the optical path of the signal light has a variable attenuation.

According to such a configuration, as long as the internal oscillation is maintained, the amplification medium is controlled by the reflectance of the optical fiber grating filters 5a and 5b and the attenuation of the optical attenuator 8 through which the oscillation light passes. Is fixed.

On the other hand, since the signal light is attenuated by the variable optical attenuator 10 during the amplification, by changing the attenuation of the variable optical attenuator 10, the total signal light gain from the input to the output of the optical amplifier can be reduced. Change.

However, changing the attenuation of the variable optical attenuator 10 does not affect the optical path of the oscillating light, so that the wavelength dependence of the gain is kept constant. Therefore, by changing the attenuation of the variable optical attenuator 10, the signal light gain can be easily changed without changing the wavelength dependence of the gain.

FIG. 2 is a block diagram showing another embodiment of the optical amplifier of the present invention.

In the present optical amplifier, like the optical amplifier shown in FIG. 1, the rare-earth-doped optical fiber is divided into two, a rare-earth-doped optical fiber 1a and a rare-earth-doped optical fiber 1b. To be excited. This optical amplifier uses two optical circulators 6c and 6d to separate the optical path between the signal light and the oscillation light between the two amplification stages by utilizing the fact that the signal light and the oscillation light propagate in opposite directions. . That is, the signal light travels from the terminal B to the terminal C of the optical circulator 6c, passes through the variable optical attenuator 10, and travels from the terminal A to the terminal B of the optical circulator 6d.

On the other hand, the oscillating light propagating in the opposite direction reaches the terminal C from the terminal B of the optical circulator 6d, and reaches the terminal B from the terminal A of the optical circulator 6c via the optical bandpass filter 7 and the optical attenuator 8a.

Here, the reason why the optical bandpass filter 7 for determining the oscillation wavelength is used in the optical path of the oscillation light is to suppress the spontaneous emission light entering the rare earth-doped optical fiber 1a from the rare earth-doped optical fiber 1b. Yes, and optical attenuator 8a
Is used to suppress the intensity of the oscillating light incident on the rare-earth-doped optical fiber 1a. With such a configuration,
A decrease in population inversion in the rare-earth-doped optical fiber 1a can be suppressed, and the noise characteristics of the optical amplifier can be improved.

According to the optical amplifier shown in FIG. 2, similarly to the optical amplifier shown in FIG. 1, by changing the attenuation of the variable optical attenuator 10 in the signal light path, the total signal light from the input to the output is changed. The gain can be changed, and at that time, the oscillation optical path is not affected, so that the wavelength dependence of the gain is kept constant. Moreover, since the two light beams are separated by using the difference in the propagation direction between the signal light and the oscillation light, the characteristic of the conventional optical amplifier shown in FIG. It is utilized as it is.

FIG. 3 is a block diagram showing another embodiment of the optical amplifier of the present invention.

The configuration of the amplifying unit is the same as that of the optical amplifier shown in FIG.

In the present optical amplifier, the amount of attenuation of the variable optical attenuator 10 in the signal light path is controlled by an electric signal by the control means. That is, this optical amplifier is an optical bandpass filter that branches a part of the amplified signal light output from the terminal C of the optical circulator 6c by the optical coupler 11 and transmits only a specific wavelength component of the signal light. The light is detected by the light receiving element 13 via 12. The current output from the light receiving element 13 and proportional to the light receiving power is converted into a voltage by the resistor R, and the voltage value and a constant reference voltage value Vref
Is amplified by the error amplifier 14, and the amount of attenuation of the variable optical attenuator 10 is controlled by the amplified output. Control means is constituted by the reference voltage value Vref, the resistance R, and the error amplifier 14.

By this feedback control system, the gain of the optical amplifier is controlled so that the signal light power detected by the light receiving element 13 is always constant. Even if the signal light input power fluctuates, a constant light output is obtained. Operation that maintains power is enabled. Here, the optical power of a specific wavelength component of the signal light is detected, and this is assumed, for example, in a case where one wave of the wavelength multiplexed signal light is used for controlling the amplifier. If the number of multiplexed wavelengths is constant and does not change, it is possible to omit the optical bandpass filter 12 and control so that the total power in the signal light band is kept constant.

As described above, according to the present invention, (1) by providing a feedback loop inside the optical amplifier and oscillating, the gain of the amplifying section is stabilized, and a constant gain is obtained regardless of the fluctuation of the input signal light power. Is obtained.

(2) To change the wavelength dependence of the gain by separating the optical paths of the signal light and the oscillation light in the middle of the amplifying unit and adjusting the attenuation of the variable optical attenuator provided in the signal light optical path. The gain can be adjusted without.

(3) By detecting the output power of the optical amplifier and controlling the variable optical attenuator so that the output power becomes constant, a constant output power can be obtained irrespective of the fluctuation of the input signal light power. In addition, it is possible to realize an optical amplifier having a stable gain wavelength dependency.

[0055]

In summary, according to the present invention, the following excellent effects are exhibited.

It is possible to provide an optical amplifier in which the gain can be easily adjusted and the wavelength dependence of the gain does not change with the gain change.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of an optical amplifier according to the present invention.

FIG. 2 is a block diagram showing another embodiment of the optical amplifier of the present invention.

FIG. 3 is a block diagram showing another embodiment of the optical amplifier of the present invention.

FIG. 4 is a block diagram showing a conventional example of an optical amplifier.

FIG. 5 is a block diagram showing another conventional example of an optical amplifier.

FIG. 6 is a diagram illustrating a difference in a characteristic of a dependency of a gain on an input signal light power depending on presence / absence of optical feedback;

[Explanation of symbols]

 1a, 1b Rare earth doped optical fiber 2a, 2b Pump light source 3a, 3b, 9a, 9b Optical multiplexer / demultiplexer 5a, 5b Optical fiber grating filter 8, 8a, 8b Optical attenuator 10 Variable optical attenuator

Claims (8)

[Claims] 1. A pumping light source, a rare-earth-doped optical fiber that amplifies light by pumping light from the pumping light source, and light in a part of a wavelength band within an amplification band is selectively fed back to the rare-earth-doped optical fiber. In an optical amplifier having feedback means,
An optical amplifier comprising: a separating / combining means for separating / combining return light and a signal light in the wavelength band in the middle of the rare-earth-doped optical fiber; and an optical attenuator for attenuating the separated signal light. 2. The optical amplifier according to claim 1, wherein said feedback means is an optical fiber grating filter disposed at both ends of said rare earth-doped optical fiber to reflect light in said wavelength band into said rare earth-doped optical fiber. 3. An output side optical coupler arranged at one end of the rare-earth doped optical fiber for taking out amplified light, and an input arranged at the other end of the rare earth-doped optical fiber to take in returned light. 2. The optical amplifier according to claim 1, comprising a side optical coupler and a band-pass filter provided between the two optical couplers and transmitting the light in the wavelength band between the two optical couplers. 4. An output optical circulator arranged at one end of the rare-earth-doped optical fiber for taking out amplified light, and an input arranged at the other end of the rare-earth-doped optical fiber to take in the returned light. 2. The optical amplifier according to claim 1, comprising a side optical circulator, and a band-pass filter provided between the two optical circulators and transmitting light in the wavelength band. 5. A separation / combination means for separating / combining the return light in the wavelength band and the signal light in the middle of the rare-earth-doped optical fiber, comprising: an input-side optical circulator disposed at one end of the rare-earth-doped optical fiber; An output-side optical circulator disposed at the other end of the rare-earth-doped optical fiber, and the return light and the signal light in the wavelength band propagate through the rare-earth-doped optical fiber in opposite directions via both optical circulators. The optical amplifier according to any one of claims 1, 3, and 4, wherein 6. A separating / combining means for separating / combining return light in the wavelength band and the signal light in the middle of the rare-earth-doped optical fiber, wherein the separation / combination means separates light in accordance with the wavelengths of the return light and the signal light. The optical amplifier according to any one of claims 1 to 4, which is an optical multiplexer / demultiplexer for combining. 7. A part of the amplified output signal light is detected,
7. The optical amplifier according to claim 1, further comprising control means for controlling an amount of attenuation of the attenuator so that the detected power becomes constant. 8. A control means for detecting the optical power of a specific wavelength component of the amplified output signal light and controlling the attenuation of the optical attenuator so that the detected power becomes constant. 7. The optical amplifier according to any one of items 1 to 6.